In gas-phase polymerization, a gaseous stream containing one or more monomers is passed through a fluidized bed under reactive conditions in the presence of a catalyst. A polymer product is withdrawn from the reactor while fresh monomer is introduced to the reactor. Residual gaseous and/or liquid components such as unreacted hydrocarbon monomer(s) and/or diluent(s) are usually absorbed in the polymer product. These volatile, unreacted monomers and/or diluents need to be removed from the polymerized particulates.
Typically the polymer product is introduced to a product separator or purge bin and contacted with a countercurrent flow of a purge gas such as nitrogen. The recovered purge gas product that includes the purge gas and volatile, unreacted monomers and/or diluents is either flared, used as fuel, or undergoes further processing to recover the valuable monomers and/or diluents. Current separation systems utilize membrane separation, adsorption materials, and/or pressure swing adsorption. While some of the valuable monomers and/or diluents are recovered, the remaining nitrogen purge gas must be flared or combusted as fuel because the concentration of monomers and/or diluents in the purge gas remains too high.
There is a need, therefore, for improved systems and methods for recovering hydrocarbons from a polymerization purge gas.